1. Field of the Invention
The present invention relates to an improvement in apparatus that serves in determining the distribution of luminance on the surface of an object, and more particularly, a picture analyzer based on multiplex optical encoding in which the analysis is of the hybrid type, or in other words is carried out simultaneously by spatial multiplexing and time multiplexing.
The basic concept common to all picture analyzing multiplex systems is to encode the N picture elements by a set of N functions as little mutually correlated as possible, i.e. by using a set of orthogonal functions. In conventional analysis systems, these functions are assimilable to narrow slots mutually off-set in time. As a result, the energy from any given picture element reaches the detector only for a short fraction of the overall analysis time. For multiplex analysis, the set of orthogonal functions used is such that the energy from a picture element goes to the detector for half the overall analysis time.
2. Description of the Prior Art
Picture multiplex analysis systems are known (of French Pat. No. 2, 140, 914 published Jan. 19, 1973 in which the set of orthogonal functions consists of pseudo-random cyclic binary functions of the type: EQU N=2.sup.k -1
Functions of this type are built up by determining the set of all the possible sequences of k bits. The starting point is any distribution of k bits carrying the value 1 and 0 (with the exception of k bits all at 1). The following distribution is obtained by eliminating the last bit and by moving the others to the sight by one unit. The first bit in the new line carries the value of the product of the last two bits in the previous line as in the truth table for the OR-exclusive function.
The operation is repeated until the initial sequence is obtained again. If this situation were to arise before all the possibilities have been exhausted (there are (2.sup.k -1)), the operation can be resumed by giving the first bit in the new sequence the value of the last bit times the anti-penultimate bit in the previous sequence.
In the case of k=3, the words obtained by this procedure are as follows: ##EQU1##
A pseudo-random sequence in the above table is selected, e.g. the sequence boxed in. Other words are obtained through circular permutation by placing the last point (i.e. point with rank N) in the first position and by shifting all the others points by one unit (the first becomes second, the second becomes third, etc . . . ). In building this way, it is therefore possible to obtain N different words which together are a set constituting the coding matrix .alpha..
______________________________________ 1 1 0 1 0 0 0 1 0 1 0 0 0 1 0 1 0 0 0 1 1 .alpha. = 1 0 0 0 1 1 0 0 0 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 0 ______________________________________
The matrix .alpha. is symmetrical. The fundamental property of the coding matrix .alpha. is written as follows: ##EQU2##
.alpha..sup.T is the transposed matrix of .alpha. and .vertline.1.vertline. is the unit matrix of rank N.
By expressing the general term for the matrix .alpha. by l.sub.j.sup.i, equation (1) may be written as: ##EQU3##
The lines in matrix .alpha. are hence not strictly orthogonal. Condition (1) is referred to as the transorthogoclity condition.
The encoding operation consists of successively superposing the encoding matrix lines on the distribution of the picture pixels. As a result of the code being cyclic in nature, it is merely a matter of shifting the encoding function by a unit step parallel to the distribution of the picture pixels at each coding operation. The encoded picture is the set provided by the measurements in the flux emerging from the distribution in the various positions of the code pattern. The number of independant measurements is therefore equal to the number of bits in the encoding function.
The encoding function is represented by the transparent and opaque zones (transmission factor 1 and 0) whose distribution gives the matrix .alpha..
Thus in the prior art, the distribution in luminance is determined on the basis of the multiplex analysis of an object picture, by implementing a picture detector and a moving grid that is designed to modulate the luminous flux emitted at each pixel in the picture by the value of the terms in a pseudo-random binary periodic function. The modulated (encoded) picture thus obtained is then demodulated (decoded) by correlating the signals the picture receiver delivers with a replica of the modulation function generated by the grid. This picture analyzing procedure reduces the effect caused by the inherent noise peculiar to the picture receiver on the luminance measurements made.
The technical progress that has been achieved is such that the noise peculiar to the picture receivers has been considerably reduced and the accuracy of the measurements obtained by this time-based multiplex analysis procedure is now limited by the noise due to the fluctuations in flux of incident photons received by the sensitive surface of the receiver.